1. Field of the Invention
This invention relates to a method and an apparatus for encapsulating material to be processed by warm isostatic pressing (WIP) or hot isostatic pressing (HIP) processes which are resorted to when producing sinters of high density by applying a high fluid pressure under high temperature condition to a green compact of ceramics powder of alumina, partially stabilized zirconia, barium titanate, zinc oxide, molybdenum disulfide or the like or a green compact of intermetallic compound powder of Ti-Al, Ni-Al or the like, or when diffusion-bonding two or more kinds of materials by application of a high fluid pressure under high temperature condition, for example, when joining overlapped sheets or plates of two or more different kinds of metals or a metallic or ceramics sheet or plate with a melt-coated ceramics or metal layer.
2. Discussion of the Background
The HIP or WIP process is a compressive forming process in which a high pressure fluid of several hundreds to several thousands kgf/cm.sup.2 is applied under high temperature condition. These processes are distinguished by high working pressure and the capability of isostatic compression as compared with other processes, for producing sinters of high density from hardly workable powdery material or for solid phase diffusion bonding. Although it is possible to generate a temperature over 2,000.degree. C. in the case of the HIP process, which employs a high pressure gas as a pressure medium, the WIP process, which uses a heat resistant oil, has an upper temperature limit at about 300.degree. C. However, in the case of high density sintering or solid phase diffusion bonding of powder material, both of these processes need a pretreatment for covering the entire work surface with a material that is capable of forming a hermetic seal.
In this connection, it is the general practice to seal the powder material in a metal capsule, as disclosed in Japanese Laid-Open Patent Applications 47-16308 and 57-116702 , which are directed to a method of pretreatment.
Exemplified in FIG. 16 is a method as proposed in Laid-Open Patent Application 47-16308, in which alternately overlapped layers of plate-like processing material 201 and metal foil 202 are placed in a metal tube 204 with a wall thickness of 3-6 mm and a welded bottom 203, and a closure lid 206 with an evacuation pipe 205 is welded to close the upper opening of the metal tube 204. Thereafter, the tube 204 is evacuated by vacuum pumping through the pipe 205 and sealed to form a capsule.
FIG. 17 exemplifies a method proposed by Laid-Open Patent Application 57-116702, in which firstly processing material 211 is placed in a tube 212 with a wall thickness of 3-6 mm in such a manner that the processing material 211 is kept out of contact with the tube 212. Then, the space between the processing material 211 and tube 212 is filled with a secondary pressure medium 213. After burying the processing material 211 completely in the secondary pressure medium 213, a closure lid 214 is circumferentially welded to a welding portion 215, and the tube 212 is evacuated by vacuum pumping through an exhaust pipe 216, closing a middle portion of the exhaust pipe 216 by fusion welding. In this case, the capsule has a greater degree of freedom in shape with respect to shape of the processing material 211 as compared with the example shown in FIG. 16 and can cope with a processing material of complicate shape.
The prior art capsules shown in FIGS. 16 and 17 have a number of problems as discussed below with reference to FIGS. 18(1) and 18(2) which show a processing material before and after a HIP treatment.
(1) The prior art capsules are formed of a metal tube or plate with a wall thickness of 3-16 mm, so that the rigidity in peripheral edge portions of the capsules (indicated by references 221 to 224 in FIG. 18(1)) differs from that of center portions (indicated by references 225 to 227 in FIG. 18(1)). Therefore, the capsules are deformed non-uniformly in the pressure elevating stage of the process, and the capsule portions with smaller rigidity are deformed prior to other portions of the capsules without undergoing deformation uniformly according to the applied pressure, hindering uniform transmission of pressure to the processing material and as a result causing distortion to the processing material. For example, as shown in FIG. 18(2), contraction into a spool-like shape occurs to the processing material. Therefore, due to the difficulty of securing flatness, these capsules are practically hardly applicable in a case where the processing material is in the form of a thin sheet of about 1 mm in thickness.
(2) When encapsulating the processing material, a difficulty is often encountered in inserting the material into a capsule, as shown in FIG. 16, in which the gap space between the processing material and the capsule is too narrow. Considering that a too broad gap space is a hindrance to the pressure transmission, it is necessary to determine the dimensions of the capsule for each processing material in such a manner as to leave a gap space of an appropriate width between the inner and outer diameters of the capsule and processing material. On the other hand, in case of the method of FIG. 17, the process is also complicated by the necessity for filling the gap space between the capsule and processing material with a secondary pressure medium which will not hinder the pressure transmission.
(3) Owing to the non-uniform deformation and distortion of the capsule under pressure and the large wall thickness of the capsule, difficulties are often encountered in taking out the processed material after the isostatic pressing treatment, necessitating to remove the capsule by means of a lathe or by the use of a strong acid. This also makes the process complicated.
(4) The seal which is formed by fuse-welding part of a rigid evacuation pipe often results in an inferior sealing condition and lacks reliability. The capsule of FIG. 16 requires the provision of means for preventing the secondary pressure medium from being suctioned at the time of gas evacuation, and therefore is disadvantageous from the standpoint of productivity.